ex post facto law

Ex post facto \Ex" post` fac"to\, or Ex postfacto \Ex"
post`fac"to\ ([e^]ks" p[=o]st" f[a^]k"t[-o]). [L., from what is
   done afterwards.] (Law)
   From or by an after act, or thing done afterward; in
   consequence of a subsequent act; retrospective.

   {Ex post facto law}, a law which operates by after enactment.
      The phrase is popularly applied to any law, civil or
      criminal, which is enacted with a retrospective effect,
      and with intention to produce that effect; but in its true
      application, as employed in American law, it relates only
      to crimes, and signifies a law which retroacts, by way of
      criminal punishment, upon that which was not a crime
      before its passage, or which raises the grade of an
      offense, or renders an act punishable in a more severe
      manner that it was when committed. Ex post facto laws are
      held to be contrary to the fundamental principles of a
      free government, and the States are prohibited from
      passing such laws by the Constitution of the United
      States. --Burrill. --Kent.
      [1913 Webster]